This invention relates to a high amylose starch-based, water resistant, alkaline corrugating adhesive with improved runnability or tack. More particularly, this invention is directed to chemically treated high amylose raw starch which is useful in alkaline corrugating adhesives and provides high levels of water resistance while being operable at high corrugator running speeds.
The term "corrugated paperboard" as used herein refers to a fluted medium and a facing adhesively joined to the tips on one or both sides of the fluted medium. The procedures employed in the production of corrugated paperboard usually involve a continuous process whereby a strip of paperboard is first corrugated by means of heated, fluted rolls. The protruding tips on one side of this fluted paperboard strip are then coated with an adhesive, and a flat sheet of paperboard, commonly known in the trade as a facing, is thereafter applied to these tips. By applying heat and pressure to the two paperboard strips thus brought together, an adhesive bond is formed therebetween. The above-described procedure produces what is known to those skilled in the art as a single-faced board in that the facing is applied to only one surface thereof. If a double-faced paperboard in which an inner fluted layer sandwiched between two facings is desired, a second operation is performed wherein the adhesive is applied to the exposed tips of the single-faced board and the adhesive-coated tips are then pressed against a second facing in the combining section of the corrugator under the influence of pressure and heat. The typical corrugating process and the operation and use of corrugators in general are described in U.S. Pat. Nos. 2,051,025 and 2,102,937 issued on Aug. 18, 1936 and Dec. 21, 1937 respectively to Bauer.
A major concern in such processes is the selection of the appropriate adhesive, as its properties will affect not only the strength and stability of the final corrugated product, but also the parameters, such as corrugator speed, within which the process can be operated. Thus, such adhesives are chosen in light of the specific requirements of the process and the properties desired in the ultimate corrugated paperboard. The adhesives most commonly used in corrugating are starch-based adhesives which are popular because of their desirable adhesive properties, low cost and ease of preparation.
The most fundamental of starch corrugating adhesives is an alkaline adhesive which is comprised of ungelatinized raw starch suspended in an aqueous dispersion of cooked starch (carrier). The adhesive is produced by gelatinizing starch in water with sodium hydroxide (caustic soda) to yield a primary mix of gelatinized or cooked carrier, which is then slowly added to a secondary mix of raw (ungelatinized) starch, borax and water to produce the fully formulated adhesive. In the corrugating process, the adhesive is applied (usually at between 25.degree. and 55.degree. C.) to the tips of the fluted paper medium or single-faced board, whereupon the application of heat causes the raw starch to gelatinize, resulting in an instantaneous increase in viscosity and tack and formation of the adhesive bond.
It is often desired or necessary in the manufacture of corrugated paperboard that the adhesive yield water resistant bonds which can withstand extended exposure to high humidity, water, melting ice and the like. A number of approaches have been devised to produce water resistant corrugating adhesives. One method involves preparation of an acidic, starch-based adhesive wherein urea-formaldehyde together with an acidic catalyst is added to the composition to produce water resistant bonds in corrugated board. Another more commonly followed method involves the use of alkaline curing starch-based adhesive wherein thermosetting resin such as urea-formaldehyde, ketone-formaldehyde or resorcinol-formaldehyde are added as crosslinking additives to produce water resistant bonds.
In recent years, due to the uncertainty of the safety of formaldehyde, efforts have been made to reduce the levels of exposure to formaldehyde or even better, to provide a formaldehyde-free alkaline starch-based adhesive composition which is useful in the manufacture of corrugated paperboard.
While water resistance is an important factor to be considered in formulating a corrugating adhesive, another property which must be considered and is directly related to the utility of the formulation is its ability to rapidly form an initial bond, termed "tack" or "green bond strength". This rate of tack development is directly related to the speed at which the corrugator can be run, i.e., runnability, and consequently there exists a need for corrugating adhesives possessing high green bond strengths and rapid development of tack.
While there are different theories regarding the respective roles of the raw starch and the carrier in the development of adhesive properties, some research has centered on the view that the carrier contributes to the bond strength and setting speed of the adhesive. Indeed, it has been suggested that good tack in the carrier leads to good tack (and therefore improved runnability) in the adhesive (see e.g., R. Williams, C. Leake and M. Silano, TAPPI, Vol. 60, No. 4, Apr. 1977, pp. 86-89). Further, it has been known for many years that a corrugating adhesive whose carrier portion is prepared from a high amylose starch exhibits superior tack to one prepared from pearl starch, which contains about 27% amylose, because the high amylose carrier will have improved rheological and film-forming properties, and also increased moisture resistance particularly when used with chemical crosslinkers. Consequently, there are many different carriers available for use in starch-based adhesives.
To date, however, relatively little experimentation has been done with the raw (uncooked or ungelatinized) starch portion of the adhesive, other than varying the species of starch source utilized. While high amylose starch, as noted above, has been used in the carrier portion of the adhesive to provide improved water resistance and tack, it has not been found to be useful in the raw starch because of its added expense and more significantly, because it does not provide the level of tack or runnability desired. This is noted in U.S. Pat. No. 3,532,648 issued on Oct. 6, 1970 where it is disclosed that by varying the secondary (unpasted) starch (i.e., ungelatinized or raw starch) employed, the properties of the adhesive can advantageously be tailored to the particular corrugated-based production requirements. The patent then discloses that when an amylose component (at least 35% by weight) is added to a given formulation to provide at least a portion of the secondary starch (raw starch), the waterproofness of the adhesive is improved but such improvement is accompanied by a decrease in the machine speed of the adhesive, depending on how much amylose component is used.
Accordingly, the need still exists for a formaldehyde-free alkaline starch-based adhesive that has good water resistance properties and also has an improved and satisfactory degree of tack or runnability that will make it particularly useful in corrugating paperboard manufacturing operations.